THE GIST

- Gecko lizards are unique in their ability to scale walls.

- Scientists have built a robot that mimics the lizard's techniques.

For years, researchers have been trying to build a robot that mimics the gecko lizard's ability to scale walls and ceilings of any texture, even glass. But duplicating the specialized biology of the lizard's feet -- which actually cling to surfaces thanks to molecular forces and not because of suction cups or toenails -- has proven difficult.

Now a Canadian research team has designed a tank-like robot that's able to roll up walls using the same molecular "clinging" technique as the gecko. The robot's tread is able to maneuver a variety of smooth and uneven surfaces, can support the whole weight of the machine and can be cleaned with a simple rinse.

"We can't do what geckos do quite yet, but we're getting closer," said Jeff Krahn, a research assistant at the Mechanisms 'N Robotics for Viable Applications (MENRVA) Lab at Simon Fraser University in Burnaby, British Columbia. He and his colleagues published their work in today's issue of Smart Materials and Structures.

A robot that can scale walls and ceilings could lead to Roomba-like machines used to clean windows on skyscrapers -- a risky job for anyone. But it could also be used to explore buildings in areas too dangerous for humans, such as those in disaster or war zones.

For a climbing robot to be practical, it needs to adhere to surfaces without leaving a gooey trail. That's why scientists are interested in dry adhesive methods, as opposed to wet adhesion, which may leave behind tacky or glue-like substances. The robot also needs to be able to traverse a wide range of surfaces. Some robots use suction, but that requires a lot of power pumping air. Other robots use claws, but those need something to grab onto. Some robots use magnets, but those only work with metal.

That's why the researchers decided to imitate the gecko, which is unique among vertebrates in its ability to scramble up vertical surfaces, no matter what the texture. Unlike animals that use claws to climb (squirrels, for instance), suction (some frogs) or even glue (a slug), a gecko sticks to walls and ceilings using the very force that attracts molecules together, called the Van der Waals force.

Each gecko's toes are covered with tiny hair-like growths called setae and each of the setae is covered with tiny structures called spatulae, each less than a micron wide. (Thousands could fit in a human hair). Molecules on the surfaces of the spatulae are attracted to molecules on the wall. On each individual spatula, the molecular attraction is very weak. But when working together, the Van der Waals forces between the molecules of the spatulae and the surface are enough to hold up the entire gecko. In fact, the force is so strong that some species of gecko could theoretically support the weight of an entire human.

Some researchers have found a way to mimic Van der Waals forces in wall-climbing robots. But they've been unable to match the sheer strength of the gecko's adhesion to surfaces. "The gecko's feet are really optimized for weight," Krahn said.

Now Krahn and his colleagues have found a way to not only make artificial setae -- that's been done -- but make then strong enough to bear the weight of the robot, more responsive to changes in surface roughness and shaped into a continuous band that works like a tank tread.

They started with the setae. The team designed polymer versions, making them 17 microns across and with ends shaped like mushroom caps. The setae are slightly flexible as well, just like the gecko's, to maximize the area that comes into contact with uneven surfaces. The more setae that contact the surface, the better it can hold the robot up.

But well-designed setae that have strong clinging power need some kind of force to peel them off. This is why Krahn and his team put the mushroom-capped setae on tank-like treads. Since treads pull away at an angle as they roll, there is no problem with the setae getting stuck.

Next was building a robot that would put enough pressure on the treads to get the sticking action. To do that, the scientists built two sections to the robot, so that as front half of the robot approaches a surface, it gets a little push from the rear half.

Since the setae and tread are clinging to the walls using molecular forces and not some kind of tacky, wet adhesive, such as glue, the machine is easier to keep clean. The treads can simply be rinsed off, which means they don't have to be replaced frequently. A wet adhesive that's picking up dust and dirt along the way would stop working -- just think of how a piece of tape gets after it's re-used many times. Geckos' feet self-clean, though nobody is quite sure how.

Mike Murphy, now at Boston Dynamics, has done extensive research on dry adhesion in robots. He said the fact that the Simon Fraser team was able to make a tread is something of a first. "Creating a continuous loop of micropatterned adhesive can be a challenge," he said. "Accomplishing internal and external transitions is a difficult maneuver for any climbing robot, but the reward is significantly improved applicability in real-world environments."